1. Field of the Invention
This invention relates, in general, to magnetic data storage media and, more specifically, to apparatus and methods for decoding self-clocking encoded data from moving magnetic storage media.
2. Description of the Prior Art
In storing and retrieving binary data on magnetic storage media, i.e., magnetic tape, discs, etc., the binary data is stored and retrieved in a timed manner by means of a clock signal which is typically stored on a separate track on the magnetic storage media to insure proper decoding of the data. However, since this requires two separate tracks, data storage capacity is limited.
Various self-clocking encoding and modulation techniques have been devised to improve the storage capability of magnetic storage media. Such techniques produce an encoded signal which is stored on a single track on the magnetic storage media and which is a combination of the binary data signal and the clock signal. Upon retrival, the clock signal is synthesized from the encoded data signal and used to decode the data from the storage media.
A common self-clocking encoding technique is termed "modified frequency modulation" (MFM). In this encoding technique, the binary state of the encoded signal is reversed at the center of a bit cell or zone for a bit cell or zone having a logic one state and at the boundary between two consecutive data bits of logic zero. While this technique is effective at increasing the data storage capacity of magnetic storage media, MFM encoded data exhibits random peak shift between the logic bit reversals in the decoded data as well as noise and variations in the velocity of the magnetic storage media which are encountered with any type of data decoding apparatus. These problems introduce variations between the phase of the clock signal and the encoded data signal which leads to decoding errors since the clock decoding pulse is not centered on each bit cell or zone but maybe offset in either advanced or delayed directions from the center of each bit cell.
A number of circuits have been devised to accurately recover MFM encoded data from magnetic storage media. However, these previous attempts have utilized relatively complex circuits incorporating precision and, accordingly, expensive components.
Thus, it would be desirable to provide a clock signal synchronization apparatus for decoding self-clocking encoded data which overcomes the problems of previously devised clock signal synchronization apparatus. It would also be desirable to provide such a clock signal synchronization apparatus which is simple and inexpensive to construct. Finally, it would be desirable to provide such a clock signal synchronization apparatus which is specifically adapted for synchronizing a clock signal with MFM self-clocking encoded data.